Answer

A new security feature in El Capitan called System Integrity Protection (https://en.wikipedia.org/wiki/System_Integrity_Protection) is preventing the current CGP installer from placing the application software in the /usr/sbin directory.

The upcoming 6.1.7 version of CommuniGate Pro will address this by installing the application software in the /usr/local directory instead.

If you can't wait for version 6.1.7, you can disable (use precaution) System Integrity Protection by rebooting OS X in Recovery Mode, launching Terminal, and issuing the following command: